1. Field of the Invention
The present invention relates to a digital watermark detection method and apparatus useful in preventing illegal copies of a digital video signal provided via, for example, a recording medium.
2. Description of the Related Art
As apparatuses for recording and playing back digital image data, such as a digital VTR, DVD (Digital Versatile Disk), and the like have prevailed, the number of digital moving images that can be played back by these apparatuses are provided. Various digital moving images are distributed via digital television broadcast via the Internet, broadcast satellite, communication satellite, and the like, enabling users to enjoy high-quality digital moving images.
It is easy to form high-quality copies from digital moving images on the digital signal level. Therefore, if some copy protection or copy control is not applied to digital moving images, there is the danger of unrestricted formation of copies of digital images. Therefore, illicit copies of digital images must be prevented, and the number of generations of copies formed by authorized users must be restricted. For this purpose, a method of appending information for copy control to each digital moving image, and preventing illicit copies or restricting copies has been proposed.
As a technique for superposing additional information to a digital moving image in such a way, digital watermarking is known. In digital watermarking, information such as identification information of the copyright owner or user of contents, right information of the copyright owner, use conditions of contents, secret information required upon using contents, the aforementioned copy control information, or the like (such information will be referred to as watermark information hereinafter) is embedded in contents of audio data, music data, moving image data, still image data, or the like, which has been converted into digital data, so as not to be easy to perceive. By detecting the embedded watermark information from the contents later as needed, copyright protection, including use control and copy control, can be achieved, and further use of the contents is possible.
As a conventional method of digital watermarking, a method that applies a spread spectrum technique is known. In this method, watermark information is embedded in a digital moving image in the following sequence.
In step E1 an image signal undergoes spread spectrum by being multiplied by a PN (Pseudorandom Noise) sequence.
In step E2, the image signal after spread spectrum undergoes frequency transformation (e.g., DCT transformation).
In step E3, watermark information is embedded in the image signal by changing the values of specific frequency components.
In step E4, the image signal undergoes inverse frequency transformation (e.g., IDCT transformation).
In step E5, the image signal undergoes inversely spread spectrum (the image signal is multiplied by the same PN sequence as in step E1).
Watermark information is detected in the following sequence, from the digital moving image, in which the watermark information has been embedded in the above sequence.
In step D1, the image signal undergoes spread spectrum by being multiplied by a PN (Pseudorandom Noise) sequence (the same PN sequence as in step E1).
In step D2, the image signal after spread spectrum undergoes frequency transformation (e.g., DCT transformation).
In step D3, the embedded watermark information is extracted from the image signal while paying attention to the values of specific frequency components.
When digital watermarking is applied to digital productions for the purpose of prevention of illicit use, a characteristic (robustness) that can prevent watermark information from being lost or tampered with, and deliberate attacks which are normally carried out on digital productions must be provided to digital watermarking. As attacks that make the watermark information of a digital image impossible to detect, cut-out, scaling (enlargement/reduction), rotation, and the like of an image are known.
When an image that has suffered such attacks is input, the conventional technique recovers synchronization of a PN sequence by executing a process for estimating a PN sequence used in step E1 at the time of embedding upon detection of watermark information. After that, the processes in steps D1 to D3 are executed to extract the embedded watermark information. However, in order to recover synchronization of the PN sequence from the image signal alone, a search must be conducted by trying a process for detecting watermark information using a plurality of candidates of PN sequences and adopting a candidate that can be detected satisfactory. For this purpose, problems of increases in arithmetic operation volume and circuit scale are posed. Further, since watermark embedded in an image signal under an attack of scaling or rotation is weakened, it is very possible that the watermark cannot be detected even if the contents (scaling, rotation, etc.) of the attack is detected and a detection method corresponding to the attack is utilized.